The  Comparative  Value  of  Different 

Specimens  of  Iodine  for  Use  in 

Chemical  Measurements 


DISSERTATION 

PRESENTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIRE- 
MENTS FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 
IN  THE  GRADUATE  SCHOOL  OF  THE  OHIO 
STATE  UNIVERSITY 


BY 


SAMUEL  MORRIS 


THE  OHIO  STATE  UNIVERSITY 
^-    1921 


The  Comparative  Value  of  Different 

Specimens  of  Iodine  for  Use  in 

Chemical  Measurements 


DISSERTATION 

PRESENTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIRE- 
MENTS FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 
IN  THE  GRADUATE  SCHOOL  OF  THE  OHIO 
STATE  UNIVERSITY 


BY 


SAMUEL  MORRIS 


THE  OHIO  STATE  UNIVERSITY 
1921 


THE  COMPARATIVE  VALUES  OF  DIFFERENT  SPECIMENS  OF 
IODINE  FOR  USE  IN  CHEMICAL  MEASUREMENTS. 

Introduction. 

This  paper1  gives  the  results  of  experiments  in  which  iodine  as  ordinarily 
prepared  as  a  standard  in  volumetric  analysis  was  compared  with  iodine 
of  atomic  weight  purity,  the  medium  of  comparison  being  a  sodium  thio- 
sulfate  solution. 

Papers  of  a  similar  sort  that  were  found  are  by  Gross2  and  Meineke.3 
Gross'  paper  is  very  brief  and  contains  none  of  the  original  data.  Meineke, 
on  the  other  hand,  prepared  a  number  of  specimens  of  iodine  by  different 
methods  and  through  the  medium  of  a  thiosulfate  solution  compared  them 
with  an  iodine  purified  by  the  method  of  Stas.  His  technique,  however, 

1  This  is  the  third  of  a  series  of  investigations  at  the  Ohio  State  Laboratory  in  which 
some  fundamental  point  in  analytical  chemistry  is  studied  by  comparing  the  substance 
or  operation  in  question  with  a  standard  substance  prepared  and  measured  with  as  near 
an  approach  to  "atomic  weight"  accuracy  as  may  be.  Other  similar  investigations  are 
in  progress  and  it  is  hoped  in  time  to  develop  new  methods  of  preparing  standard  sub- 
stances and  new  apparatus  for  increasing  the  accuracy  of  the  important  analytical 
measurements.  The  other  papers  are  in  J.  Am.  Chem.  Soc.,  36,  2360  (1914),  and  40, 
1664  (1918). 

1  Gross,  J.  Am.  Chem.  Soc.,  25,  987  (1903). 

8  Meineke,  Chem.  Ztg.,  16,  1126.  1219,  and  1230  (1892). 


492858 


was  that  of  an  accurate  analytical  procedure,  rather  than  of  a  precise 
measurement,  and  therefore  it  seemed  worth  while  to  supplement  his 
results  by  the  scheme  followed  in  this  paper.  Meineke's  papers  are  rich 
in  information  about  the  purification  of  iodine  and  contain  a  wealth  of 
experimental  data. 

Preparation  of  Materials. 

Water. — "Conductivity"  water  was  used  throughout. 

Starch  Indicator. — A  sterilized  starch  paste  made  by  the  method  given  in  Tread- 
well's  "Quantitative  Analysis"  was  employed. 

Potassium  Iodide. — One  of  the  so-called  analyzed  brands  was  used.     Tests  showed 
that  no  iodine  was  set  free  from  its  solutions  under  the  conditions  of  this  work. 

Hydrogen  Sulfide. — The  gas  was  generated  from  ferrous  sulfide  with  c.  P.  sulfuric 
acid.     Before  use  it  was  washed  5  times  with  water. 

Potassium  Permanganate. — The  c.  p.  salt  was  recrystallized  twice  in  platinum. 
It  gave  no  reaction  for  chlorine. 

The  Various  Specimens  of  Iodine. — "ATOMIC  WEIGHT"  IODINE.  One  of  the 
general  methods  of  Baxter4  was  faithfully  followed,  excepting  as  noted  below,  the  cycle 
of  purification  processes  being  carried  through  three  times.  The  exceptions  were 
(a)  a  modification  of  the  apparatus  for  distilling  iodine  from  a  liquid  mixture  and  (6) 
the  apparatus  for  the  final  drying  of  the  iodine  at  the  conclusion 
of  the  wet  processes  of  purification.  The  distillation  apparatus 
is  illustrated  in  Fig.  1.  The  essential  feature  is  the  condensation 
tube,  b,  about  19  mm.  in  diameter  and  long  enough  to  project  8 
or  10  cm.  into  the  GOOcc.  Erlenmeyer  flask,  a.  It  is  cooled  by  a 
stream  of  tap  water,  passing  through  it  as  shown  in  the  drawing 
and  is  held  in  position  by  a  bulb  blown  near  the  top.  This  bulb  is 
given  an  umbrella  shape  which  serves  for  closing  the  neck  of  the 
flask  against  loss  of  iodine  and  also  for  protecting  it  against  falling 
dust  or  other  contamination.  By  heating  the  mixture  in  the  flask 
almost  to  boiling  the  iodine  distils  and  solidifies  on  the  condensa- 
tion tube.  There  is  no  contamination  due  to  bumping  with  con- 
sequent projection  of  liquid  onto  the  iodine.  A  slight  danger  of 
splashing  from  falling  drops  of  water  that  had  condensed  on  the 
cold  tube  is  eliminated  by  placing  a  narrow  glass  plate  as  shown 
at  e.  The  drops  strike  this  and  glide  down  gently.  This  ap- 


Fig.  1. 


paratus,  which  is  analogous  to  one  described  in  Treadwell's  "Quantitative  Analysis," 
has  proved  of  great  value  in  the  distillation  of  iodine.  Drying  and  sublimation  of  the 
iodine  was  effected  in  an  apparatus  made  from  a  Hempel  desiccator  modified  by  inserting 
supports  for  carrying  a  number  of  watch  glasses,  and  by  grinding  in  a  condensation  tube 
which  projected  through  the  tubulure  at  the  top.  Phosphorus  pentoxide  was  placed  on 
the  watch  glasses  and  in  the  circular  tray  of  the  desiccator.  Iodine  to  be  dried  is  placed 
in  a  dish  in  the  bottom  of  the  apparatus,  which  is  moderately  warmed.  The  temperature 
of  the  interior  at  a  point  about  midway  between  the  top  and  bottom  was  40  °.  Under 
these  conditions  iodine  vapor  ascends  slowly  over  and  around  the  phosphorus  pentoxide 
and  solidifies  on  the  condensation  tube,  which  is  cooled  with  tap  water.  Like  the  dis- 
tillation apparatus,  this  drying  tower  was  found  to  have  many  advantages.  Its  use  saved 
time  because  it  needed  no  attention  while  in  operation. 

As  a  final  note,  it  should  be  stated  that  all  transfers  of  the  "atomic  weight"  iodine 


4  Baxter,  J.  Am.  Chem.  Soc.,  26,  1579  (1904). 


from  one  vessel  to  another  that  occurred  in  the  last  of  the  series  of  purification  operations 
were  made  in  a  dust-proof  box  equipped  with  rubber  sleeves. 

IODINE  2. — Commercial  iodine  was  ground  with  potassium  iodide  and  sublimed, 
and  the  sublimation  repeated  without  the  use  of  potassium  iodide.  The  final  product 
was  then  ground  in  a  mortar  and  placed  over  calcium  chloride. 

IODINE  3. — This  was  commercial  "resublimed"  iodine  just  as  received  from  the 
laboratory  store-room. 

IODINE  4. — Iodine  recovered  from  titration  residues  and  from  residues  from  the 
purification  of  iodine  by  the  Baxter  method  was  the  starting  point  of  this  preparation. 
When  free  iodine  was  present  in  these  residues  it  was  reduced  by  hydrogen  sulfide  and 
the  resulting  hydriodic  acid  neutralized  by  sodium  hydroxide.  The  mixture  was  then 
evaporated  in  the  open  air,  treated  with  manganese  dioxide  and  sulfuric  acid  and  the 
iodine  distilled.  This  iodine  was  then  twice  dissolved  in  potassium  iodide  solution  and 
redistilled.  It  was  dried  in  a  vacuum  desiccator  over  calcium  chloride  for  one  week. 

IODINE  5. — This  was  the  same  as  Iodine  4,  except  that  it  was  dried  in  the  phosphorus 
pentoxide  apparatus. 

IODINE  6. — This  was  prepared  in  all  respects  like  the  "atomic  weight"  iodine, 
excepting  that  it  was  not  sublimed  in  the  phosphorus  pentoxide  apparatus.  The  product 
from  the  last  distillation  from  potassium  iodide  solution  was,  instead,  placed  over  sul- 
furic acid  where  it  remained  for  93  days  before  it  was  used. 

Comparison  of  the  Various  Specimens  of  Iodine. 

Balance  and  Weights. — A  long-armed  Troemner  balance  and  a  set  of  gold-plated 
brass  weights  with  the  fractional  parts  of  platinum  were  used.  These  weights  had 
been  calibrated  by  the  Bureau  of  Standards.  All  weighings  were  made  at  night  in 
order  to  avoid  the  vibrations  of  the  building.  The  method  of  tares  was  used  throughout 
and  it  is  believed  that  the  weighings  are  defined  to  within  0.02  to  0.03  mg.  Since  there 
is  no  reason  to  suppose  that  the  various  specimens  of  iodine  differed  materially  in  density, 
weights  were  not  reduced  to  a  vacuum,  but  they  were  corrected  for  the  different  densities 
of  brass  and  platinum,  so  that  the  values  as  finally  given  are  those  that  would  have  been 
obtained  had  all  of  the  weights  been  of  platinum. 

Preparation  of  the  Iodine  for  Weighing. — 1.  In  case  the  iodine  was  to 
be  fused  it  was  placed  in  a  porcelain  boat  in  the  heating  tube  of  a  Richards 
bottling  apparatus.6  After  passing  dry  air  over  the  sample  for  20  to 
30  minutes,  the  part  of  the  tube  occupied  by  the  boat  was  cautiously  heated 
until  the  iodine  just  melted.  With  care  the  iodine  could  be  maintained  in 
the  liquid  state  for  one  or  two  minutes  without  undue  loss  by  volatilization. 
The  usual  procedure  was  to  keep  it  fused  for  one  minute  and  then  push 
the  boat  into  a  cooler  part  of  the  tube  towards  the  bottling  chamber. 
The  bottom  of  the  boat  being  then  in  contact  with  cold  glass  cooled  while 
the  upper  part  remained  warm.  Iodine  that  had  sublimed  on  the  edges 
of  the  boat  volatilized  again,  so  that  in  the  end  the  only  iodine  visible  was; 
the  hard,  glassy  looking  cake  in  the  bottom  of  the  boat.  During  the  whole 
operation  the  current  of  dry  air  was  maintained  and  continued  until  every- 
thing was  completely  cool.  The  boat  was  then  bottled  and  after  lying 
in  the  balance  case  for  at  least  an  hour  was  weighed.  The  weighing  bottle 
and  empty  boat  had  previously  been  put  through  the  same  process  at 
8  Richards  and  Willard,  J.  Am.  Chem.  Soc.,  32,  25  (1910). 


6 

least  twice  or  until  constant  weight  was  reached.  In  the  weighing  of  all 
samples  a  weighing  bottle  and  boat  as  nearly  as  possible  like  those  con- 
taining the  iodine  were  used  as  a  counterpoise. 

The  air  passing  through  this  bottling  apparatus  was  first  filtered  through 
a  2-liter  bottle  filled  with  cotton  wool.  It  next  passed  through  an  all-glass 
series  of  purifying  and  drying  towers,  the  first  one  of  which  contained 
glass  beads  over  which  silver  nitrate  solution  trickled,  and  the  second  was 
filled  with  sodium  carbonate  that  had  previously  been  fused  and  the 
fusion  cake  broken  into  pieces;  next  followed  4  towers  of  beads  over  which 
cone,  sulfuric  acid  was  kept  flowing,  and  finally  a  tube  filled  with  alternate 
layers  of  glass  wool  and  phosphorus  pentoxide. 

2.  When  the  sample  of  iodine  was  not  to  be  fused,  it  was  transferred 
from  its  container  to  the  weighing  bottle  as  quickly  as  possible  and  with 
cautions  against  contamination  from  dust. 

Sodium  Thiosulfate  Solution. — The  solution  of  sodium  thiosulfate  used 
throughout  the  work  was  a  residuum  of  5  or  6  liters  from  a  larger  lot  that 
had  been  made  up  in  December,  1918,  and  standardized  in  January,  1919, 
by  titrating  it  from  a  volume  buret  against  resublimed  iodine  prepared  as 
described  above  for  Iodine  2.  Its  value  then  was  0.1003  N.  During  the 
interval  of  2  years  the  solution  had  stood  in  an  uncolored  glass  bottle 
exposed  to  the  light  but  protected  by  a  soda-lime  tube  against  acid  fumes 
and  carbon  dioxide.  When  it  assumed  a  rdle  in  this  investigation  it  was 
placed  in  a  dark  room.  The  first  hope  that  a  thiosulfate  solution  as  old 
as  this  had  become  constant  was  unfounded,  for  a  few  months'  work  showed 
a  progression  in  the  results  that  pointed  to  a  gradual  decrease  in  its  iodine 
value.  Accordingly,  in  the  following  table  the  final  thiosulfate  values  of 
the  various  specimens  of  iodine  are  calculated  to  the  value  which  the  thio- 
sulfate solution  had  on  April  10,  1921. 

The  correction  factor  was  calculated  from  the  average  thiosulfate  solution 
values  of  0.40000  g.  of  "atomic  weight"  iodine  as  found  in  Groups  I  and 

II,  and  from  the  corresponding  values  of  Iodine  2,  as  found  in  Groups 

III,  IV,  and  V  in  the  table.    This  gives  three  comparable  time  intervals, 
which  coincide  well  with  the  whole  work,  and  from  the  data  the  average 
daily  change  in  this  thiosulfate  solution  in  terms  of  weight  of  solution 
to  react  with  0.40000  g.  of  iodine  was  found  to  be  0.000787  g.     This  amount 
multiplied  by  the  time  in  days  between  the  date  of  an  experiment  and  April 
10,  was  added  or  subtracted,  as  the  date  demanded.     The  authors  are,  of 
course,  aware  that  such  a  procedure  is  not  to  be  recommended  in  precise 
measurements,  but  in  their  case  it  became  a  necessity  in  order  to  save  the 
results.     The  error  introduced  does  not  destroy  the  value  of  the  results  as  a 
contribution  to  analytical  chemistry 

Solution  and  Titration  of  the  Iodine. — Two  titration  flasks  were 
employed  similar  in  all  respects  excepting  that  one  of  them  was  closed  with 


a  rubber  stopper  and  the  other  one  was  entirely  of  glass  as  illustrated 
by  Fig.  2.  The  ground-in  stopper  carried  2  tubes,  a,  for  introducing  the 
sodium  thiosulfate  solution  and  b,  for  the  escape  of  displaced  air.  Loss 
of  iodine  vapor  during  a  titration  was  prevented  by  having  Tube  a,  dip 
below  the  surface  of  the  liquid  and  by  potassium  iodide  solution  in  the 
bends  of  Tube  b. 

The  introduction  of  the  weighed  portions  of  iodine  into  this  titrating 
flask  was  accomplished  by  first  loosening  but  not  removing  the  stopper 
of  the  weighing  bottle  and  then  putting  the  bottle  into  the  flask.     Next 
with  the  flask  in  an  inclined  position  the  stopper  of  the  weighing  bottle 
was  removed  with  a  glass  hook  which  was  quickly  withdrawn  and  the 
flask   closed.     Two   cc.  of  saturated  potassium  iodide 
solution  was  then  added  through  Tube  a,  followed  by 
a  little  water.     By  properly  rotating  and  inclining  the 
titrating  flask  the  boat  was  brought  out  of  the  weighing 
bottle  so  that  all  surfaces,  that  is,  of  the  weighing  bottle, 
its  stopper,  which  had  been  left  in  the  flask,  and  of  the 
boat,  could  be  bathed  in  the  strong  potassium  iodide 
solution.     When  the  iodine  was  dissolved,  the  volume 
was  made  up  to  about  150  cc.  with  water  and  was  ready 
for  titration.     This  was  done  according  to  the  following 
routine.     Thiosulfate  solution  was  added    in   approxi- 
mately 5g.  portions  with  intermediate  mixing  by  shak- 
ing  the  flask.    When  all  but  a  trace  of  iodine  had  been  re- 
duced, starch  indicator  was  added  and  the  titration  finished. 

In  order  to  reach  a  uniform  end-point  4  comparison  flasks  of  the  same 
size  and  color  of  glass  as  the  titrating  flask  were  prepared  and  charged  with 
solutions  in  every  way  like  that  in  the  titration  flask  at  the  end  of  a  titration 
excepting  in  respect  to  the  content  of  iodine.  Comparison  flask  No.  1, 
contained  no  iodine  and  Nos.  2,  3  and  4  had  amounts  of  iodine  equivalent 
respectively,  to  0.006  g.,  0.012  g.,  and  0.018  g.  of  the  thiosulfate  solution. 
These  varying  amounts  of  iodine  gave  with  starch  a  set  of  color  standards 
by  which  it  was  easy  to  follow  the  approach  of  the  end-point  in  the  solution 
being  titrated.  In  any  event  each  end-point  was  tested  by  titrating  back 
with  an  iodine  solution. 

It  was  found  that  a  more  uniform  end-point  could  be  obtained  with  the 
light  from  an  ordinary  tungsten  bulb  than  with  the  varying  light  of  the  sun. 
The  bulb  was  supported  a  few  centimeters  above  the  desk  top  with  a  sheet 
of  white  paper  above  it  to  reflect  the  light  downwards  upon  other  white 
paper  on  the  desk.  Upon  this  white  surface  on  the  desk  were  placed  the 
color  standard  flasks  and  from  time  to  time  the  titrating  flask  for  compari- 
son. The  observations  were  made  from  a  point  above  the  light  bulb. 
In  this  way  the  end-points  were  reproducible  within  0.006  g.  of  the  thio- 


8 

sulfate  solution.  Another  reason  for  preferring  artificial  light  to  that  of  the 
sun  is  the  action  of  the  latter  in  liberating  iodine.  Concentrated  solutions 
of  potassium  iodide  became  distinctly  yellow  in  10  minutes  in  direct  sun- 
light. 

TABLE  I 
ANALYTICAL  RESULTS 


Iodine 
G. 

Iodine 
G. 

Thio- 
sulfate 
solution 

G. 

Thio-    Average  Average  Purity  of 
sulfate                  corrected    iodine 
solution               for  change 
calculated                in  thio- 
to  0.40000                 sulfate 
iodine                   solution 
G.              G.              G.                % 

Group    I,    April    10,    1921, 

0  .39206 

32 

,3748 

33 

.030 

"Atomic  Weight"  Iodine, 

0 

.45868 

37 

.8628 

33 

.019 

not  fused. 

0 

.48325 

39 

.8879 

33 

.014    33.030    33.030 

100.000 

0 

.49642 

41 

.0163 

33 

.050 

0 

57185 

47 

.2322 

33 

.038 

Group    II,    April   29,    1921, 

0 

.54560 

45 

.0777 

33.049 

"Atomic  Weight"  Iodine, 

0 

.33932 

28 

,0087 

33 

.018 

fused. 

0 

.40043 

33 

.0710 

33 

.035    33.045    33.030 

100.000 

0 

.57602 

47 

.6233 

33 

.071 

0 

,49461 

40 

.8684 

33 

.051 

Group   III,   Jan.    18,    1921, 

0 

.37134 

30 

.6151 

32 

.978 

Iodine  2,  resublimed  once 

0 

.46267 

38 

.1505 

32 

.983 

from   KI  and  once  alone; 

0 

.55278 

45 

.6171 

33.009 

ground     and     kept     over 

0 

.46183 

38 

.0386 

32 

.946    32.973    33.038 

100.024 

CaCl2. 

0 

.42732 

35 

.1963 

32 

.946 

0 

.41060 

33 

.8484 

32 

.975 

0 

.44523 

36 

.7008 

32 

.972 

Group   IV,   Feb.    16,     1921, 

0 

.54446 

44 

.9246 

33 

.005 

Iodine     2,     as     described 

0 

.52526 

43 

.3212 

32 

.991    32.997    33.039 

100.018 

above. 

0 

.46477 

38 

.3382 

32 

.995 

0 

.46002 

37 

.9478 

32 

.995 

Group  V,  April  20,    1921, 

0 

.49583 

40 

.9530 

33 

.038 

Iodine     2,     as     described 

0 

.49537 

40 

.9209 

33 

.043     33.044     33.036 

100.018 

above. 

0 

.51626 

42 

.6567 

33 

.051 

0 

,46010 

38 

0061 

33 

042 

Group  VI,    Feb.    17,     1921, 

0 

.26374 

21 

.7366 

32 

.968 

Iodine  3,  product  just  as 

0. 

36245 

29. 

9089 

33. 

008    32.995    33.036 

100.018 

received  from  store-room, 

0 

.34615 

28 

.5655 

33 

.009 

except  that  it  was  fused 

before  weighing. 

Group  VII,  Feb.  23,    1921, 

0 

.35231 

29 

.0509 

32 

.984 

Iodine  3,  as  above,  but  not 

0  .40486 

33 

.4021 

33 

.002 

fused  before  weighing. 

0 

.38186 

31 

.4888 

32 

.984    32.994    33.030 

100.000 

0 

.43344 

35 

.7709 

33 

.011 

0 

.41145 

33 

.9214 

32 

.977 

0 

.46869 

38 

.6702 

33 

.003 

g 


Group  VIII,  April  21,  1921, 
Iodine  3,  as  above,  but  not 
fused. 

Group  IX,  March  17,  1921, 
Iodine  4,  recovered  from 
residues;  dried  over  CaCU 
for  one  week  but  not  sub- 
limed. 

Group  X,  March  22,  1921, 
Iodine  5,  same  as  No.  4, 
excepting  that  it  was  sub- 
limed in  PjOj  apparatus. 

Group  XI.  Feb.  14,  1921, 
Iodine  6,  "Atomic 

Weight"  Iodine;  not  sub- 
limed but  kept  93  days 
over  H2SO4. 

Group  XII,  May  5,  1921, 
"Atomic  Weight"  Iodine 
after  7  hours  exposure  in 
current  of  air  saturated 
with  water  vapor. 


0.45876 
0.56198 
0.49452 
0.50921 


37.8893 
46.4199 
40.8364 
42.0799 


33.036 
33.041 
33.031 
33 .055 


0.45363    37.3096    32.899 
0.28391     23.3198    32.854 


0 

.36848 

30 

.4221 

33 

.024 

0 

.39392 

32 

.5201 

33 

.022 

0 

.46061 

38 

.0158 

33 

.014 

0 

.38952 

32 

.1678 

33 

.033 

0 

.53442 

43 

.9857 

32 

.921 

0 

.40481 

33 

.3249 

32 

.929 

0 

.42985 

35 

.3811 

32 

.924 

0 

42995 

35 

.4024 

32 

.936 

n 

.42324 

34 

.9834 

33 

.062 

0 

.50817 

41 

.9983 

33 

.058 

0 

.54587 

45 

.1022 

33 

.050 

0 

.49967 

41 

.2612 

33 

.031 

0 

.43242 

35 

.7428 

33 

.063 

33.041     33.032     100.006 


32.877    32.889      99.57 


33.023    33.038     100.024 


32.928    32.971      99.82 


33.053    33.033     100.009 


Discussion  of  Results. 

With  the  exception  of  Nos.  4  and  6,  the  specimens  of  iodine  examined 
agreed  within  0.02%  with  the  "atomic  weight"  iodine.  The  deviation 
from  the  "atomic  weight"  iodine,  except  as  noted,  was  always  above 
100%,  which  would  seem  to  indicate  such  impurities  as  chlorine  and 
bromine.  This,  however,  is  unlikely,  since  the  unpurified  iodine,  No.  3, 
has  a  smaller  positive  error  than  the  others.  It  would  also  seem  from  the 
nature  of  its  preparation  that  Iodine  5  would  hardly  be  contaminated 
that  way.  The  cause  is  seemingly  in  a  systematic  error  that  has  escaped 
detection. 

Evidence  of  water  in  No.  4  is  found  in  the  fact  that  it  rose  in  value  when 
put  through  the  phosphorus  pentoxide  drying  tower. 

The  low  value  of  No.  6  could  be  accounted  for  on  the  basis  of  a  statement 
by  Treadwell,6  that  iodine  takes  up  sulfuric  acid  when  dried  over  it. 
This  statement,  however,  can  hardly  be  accepted  in  the  light  of  evidence 
to  the  contrary  in  the  literature.  Baxter7  dried  some  of  his  atomic  weight 
iodine  over  sulfuric  acid  and  makes  no  mention  of  it  as  a  possible  impurity 
that  was  removed  in  the  subsequent  sublimation  of  the  iodine.  Meineke8 
reports  several  experiments  in  which  iodine  was  exposed  for  days  over 

'  Treadwell,  "Quantitative  Analysis,"  John  Wiley  and  Sons  Co.,  3rd  Ed.,  p.  646. 

7  Baxter,  /.  Am.  Chem.  Soc.,  32,  1594  (1910). 

8  Meineke,  Ref.  3,  pp.  1126,  792. 


10 

sulfuric  acid  without  evidence  that  any  of  the  acid  was  taken  up  by  the 
iodine. 

It  is  the  belief  of  the  authors  that  Iodine  6  still  contained  water  in  spite 
of  its  long  exposure  over  sulfuric  acid.  An  inspection  of  Nos.  4  and  6 
shows  that  they  had  one  point  in  common,  namely,  distillation  from  water 
and  condensation  on  a  cold  surface  on  which  water  was  condensing  at  the 
same  time  in  contact  with  the  iodine.  This  condensation  in  the  wet  way 
was  the  last  purification  process,  except  the  drying  (?)  over  a  desiccating 
agent  to  which  Nos.  4  and  6  were  subjected.  No.  4,  undoubtedly  con- 
tained water  (see  above).  Lack  of  time  and  material  made  it  impossible 
to  test  No.  6  directly,  and  until  the  point  can  be  investigated  experiment- 
ally, the  hypothesis  is  offered  that  iodine  when  it  solidifies  in  the  presence 
of  liquid  water  entrains  some  of  the  water  in  a  way  that  withstands  months 
of  exposure  over  a  desiccating  agent.  The  complete  removal  of  water 
from  iodine  demands  dry  sublimation  in  the  presence  of  a  desiccating 
agent. 

Is  Iodine  Hygroscopic? — Meineke9  exposed  dry  iodine  for  5  days  to 
the  air  of  his  laboratory  and  reports  that  the  change  observed  could  be 
ascribed  to  experimental  errors.  Powdered  iodine  exposed  under  a  bell 
jar  with  a  dish  containing  water  took  up  0.09%  of  water  in  48  hours.  Iodine 
crystals  when  subjected  to  the  same  conditions  took  up  onlyO  .05%  of  water 
in  5  days.  An  exposure  of  10  days  over  sulfuric  acid  brought  these  last 
two  specimens  back  to  100.00%.  Meineke's  iodines  had  all  been  sublimed 
in  the  dry  way  and  therefore  the  situation  is  not  comparable  with  that  of 
No.  6,  above. 

In  this  investigation,  the  following  experiment  was  made  to  test  the 
hygroscopic  nature  of  iodine.  About  5  g.  of  the  "atomic  weight"  iodine 
was  spread  in  a  thin  layer  in  a  glass  tube  and  a  current  of  air  previously 
bubbled  through  water  was  passed  over  it  for  7  hours  at  the  rate  of  about 
9  liters  an  hour.  The  results  are  given  in  Group  XII,  of  the  table,  and  show 
that  if  any  water  was  taken  up,  the  amount  was  well  within  the  experi- 
mental errors.  This  experiment  and  those  by  Meineke  show  that  iodine 
is  not  hygroscopic  to  a  degree  that  would  affect  analytical  work. 

A  Possible  Contribution  to  the  Data  of  Adsorption. — One  of  Meineke's 
experiments  shows  that  when  iodine  is  finely  divided  and  exposed  to  air 
saturated  with  water  vapor,  a  small  amount  of  water  is  taken  up.  It  is 
reasonable  to  suppose  that  this  water  is  adsorbed  on  the  surface  of  the  solid 
iodine,  and  the  question  then  arises  as  to  what  effect  the  relatively  high 
vapor  pressure  of  the  iodine  has  on  such  adsorption.  If  adsorption  is  a 
surface  phenomenon,  then  a  surface  like  that  of  an  iodine  crystal  which  is 
constantly  sending  off  molecules  and  therefore  constantly  presenting  a 
fresh  surface  must  introduce  complications.  The  rate  of  adsorption  under 
»  Ref.  3,  p.  1126. 


11 

such  conditions  would  be  slow  and  if  later  such  a  solid  were  placed  in  an 
atmosphere  free  from  water  vapor  the  portion  adsorbed  would  soon  be 
given  up  because  the  surface  holding  it  would  have  disappeared.  One 
would  therefore  expect  solids  with  high  vapor  pressures  to  be  slightly  or 
not  at  all  hygroscopic. 

The  Use  of  a  Rubber  Stopper  in  the  Titrating  Flask. — The  titrations  of  Group  VII 
were  made  in  a  flask  closed  with  a  rubber  stopper,  and  those  of  the  strictly  comparable 
Table  VIII  were  carried  out  in  the  all-glass  apparatus.  The  manipulation  was  the 
same  in  both  cases.  The  agreement  between  these  two  sets  of  results  is  almost  perfect, 
and  therefore  the  use  of  a  rubber  stopper  may  be  permitted  in  analytical  operations. 

Summary. 

Some  new  forms  of  apparatus  for  the  preparation  and  handling  of  pure 
iodine  are  described. 

Various  specimens  of  purified  iodine  were  compared  with  a  highly  purified 
iodine.  The  agreement  was  found  to  be  close;  0.024%  was  the  maximum 
deviation. 

Experiments  are  described  which  show  that  a  rubber  stopper  may  be 
used  for  closing  the  flask  in  which  iodine  is  titrated. 

The  usual  method,  as  given  in  text-books  of  analytical  chemistry, 
of  drying  iodine  by  exposing  it  in  a  desiccator  with  a  drying  agent,  is  ques- 
tioned if  the  iodine  had  previously  solidified  in  the  presence  of  liquid  water. 

The  hygroscopic  properties  of  iodine  are  discussed. 

A  suggestion  is  made  that  solids  like  iodine  that  have  a  measurable 
vapor  pressure  at  room  temperature  possess  peculiar  adsorptive  properties 
due  to  the  fact  that  a  fresh  surface  is  being  continually  exposed. 

The  author  wishes  to  express  his  thanks  to  Professor  C.  W.  Foulk  for 
his  help  and  untiring  interest  throughout  this  work. 


AUTOBIOGRAPHY. 

I,  Samuel  Morris,  was  born  near  Bloomingburg,  Ohio,  October  20,  1879. 
I  received  my  secondary  school  education  in  the  public  schools  of  Fayette 
County  Ohio,  graduating  from  Bloomingburg  High  School  in  1898.  My 
undergraduate  and  graduate  work  was  all  done  at  the  Ohio  State  Uni- 
versity, receiving  the  Degree  of  Bachelor  of  Arts  in  1905,  the  Degree  of 
Master  of  Arts  in  1908  and  the  Degree  of  Doctor  of  Philosophy  in  1921. 


YD  04904 


JL/M& 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


